1. Field of the Invention
The invention relates to an improved finger structure, and more particularly, to an improved finger structure applied to a packaging stack structure.
2. Description of the Prior Art
For satisfying the requirement of the multi-function system with high complex, high density and thin thickness, the packaging stack structure technology is gradually developed and the multi-chip packaging stack structure is also widely utilized. The chip packaging stack technology is the technology directly stacks an IC chip on another, and the IC chips are electrically connected and packaged together. This technology can improve the capability and reduce the size.
FIG. 1 is a schematic diagram of a conventional packaging stack structure. The packaging stack structure 10 has the upper and lower chips 102, 103 with different functions. The upper chip 102 and the lower chip 103 have several leading wires 104 connecting to the finger 106, and the design of the finger 106 is shape of regular rectangle or ellipse. When the upper chip 102 and the lower chip 103 are electrically connected with the same finger 106, that also means several leading wires 104 are bonded to the same finger 106, a process issue will occur. The conventional bonding process uses a obverse bonding, that also means the upper and lower chips 102, 103 are bonded to the finger 106 along the direction shown in FIG. 2, which is a partially magnified schematic diagram of the finger 106 with the obverse bonding. The bonding direction should be designed along the upper and lower chips 102, 103 to the finger 106. When finishing the welding point on the finger 106 in the obverse bonding process, the leading wire 104 should be pressed with the steel stamp 108 and produces the fishtail structure 110 on the leading wire 104. A press distance for the steel stamp 108 is required on the finger 106.
For solving the above-mentioned problem, two ways are generally used. The first way is enlarging size of the finger 106, as shown in FIG. 3. However, since the layout space is limited and the enlarged size of the finger 106 is also limited. Another way is using the reverse bonding process, that also means the wire is bonded from the finger 106 to the chip. But the problem is that the reverse bonding process isn't a popular process in a general packaging factory, and the reverse bonding process needs forming the bump on the chip surface and then bonding from the finger 106 to the bump. The extra procedure of forming the bump will cause the process and reliability issues. For example, when bonding with hanging in the air, the chip has no support under it, and the striking strength on the chip using the reverse bonding process is larger than that using the obverse bonding process.
Hence, the present invention discloses an improved finger structure to solve the above-mentioned problem.